Bottom Line:
Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities.We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils.These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

ABSTRACTBone is a natural composite of collagen protein and the mineral hydroxyapatite. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale. Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities. We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils. Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material's deformation response. These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

f2: Mineral distribution in the collagen microfibril at different mineralization stages.(a) Distribution of HAP along the collagen fibril axis. The data shows that the maximum amount of HAP is found in the gap region (between 30 and 50 nm). (b) Spatial distribution of HAP in the unit cell for 20 and 40% mineral density. (c) HAP density distribution along the fibril axis for the 40% case normalized (same data as depicted in panel a) compared with experimental data26. The comparison confirms that maximum deposition is found in the gap region.

Mentions:
We apply an in silico mineralization scheme based on a Monte Carlo approach, and identify molecular structures of nascent bone for different mineral densities, from 0% (pure collagen microfibril, see Fig. 1b for model details) to 40% (highly mineralized collagen fibril). As shown in Fig. 2a, the mineral is deposited predominantly in the gap region along the fibril axis, with sparse deposition in the overlap region. Figure 2b shows a detailed analysis of the distribution of mineral crystals for 20 and 40% mineral densities. We perform a direct comparison with experimental results that reported the distribution of mineral density as a function of the fibril axis26, and plot the mineral distribution of 40% case normalized to its maximum value along with experimental results along the fibril axis (Fig. 2c). In good agreement with a rich set of experimental data1234562627 in the mineralized microfibril models with varying densities (5, 10, 20, 40%, and so on), the mineral deposition occurs primarily in the gap region. This observation is also consistent with the experimental finding26 that the mineral nucleation point is close to the carboxy terminus (in the first section of the gap region, immediately after the gap/overlap transition). As discussed by Nudelman et al.26, the main reason is the high concentration of positive charge, which attract negatively charged peptides, in turn responsible for onset of mineralization. Our in silico mineralization process is not driven by electrostatic interactions (see Methods), but rather relies on a geometric argument, driven by the void space available within the virgin (non-mineralized) collagen fibril. This may suggest that in the in vivo mineralization process there could be concurrent mechanisms that lead to the nucleation in the C-terminal region. Along with the concentration of positive net charges, the C terminus region also features larger void spaces in the collagen packing, which makes this region the preferred site for mineral nucleation. We find that the 40% mineral-density case shows more mineral deposition in the gap region compared with experimental data26. This could be due to the fact that in the experimental work, the mineral density profile is measured for collagen that is mineralized for 24 h. Our model shows that the size of the mineral platelets in the gap region is ~15 × 3 × 1.6 nm3 (for the case of 40% mineral density). This is in good agreement with experimental work that has shown that the size of mineral platelets is 15–55 × 5–25 × 2–3 nm3 (262728). We note that the models are based on a periodic unit cell of a collagen microfibril with approximate dimensions of 67.8 × 4 × 2.7 nm (see further discussion regarding structural aspects below). It is possible that the mineral phase can span several unit cells and, hence, forms larger size HAP platelets in vivo, explaining the larger range of values.

f2: Mineral distribution in the collagen microfibril at different mineralization stages.(a) Distribution of HAP along the collagen fibril axis. The data shows that the maximum amount of HAP is found in the gap region (between 30 and 50 nm). (b) Spatial distribution of HAP in the unit cell for 20 and 40% mineral density. (c) HAP density distribution along the fibril axis for the 40% case normalized (same data as depicted in panel a) compared with experimental data26. The comparison confirms that maximum deposition is found in the gap region.

Mentions:
We apply an in silico mineralization scheme based on a Monte Carlo approach, and identify molecular structures of nascent bone for different mineral densities, from 0% (pure collagen microfibril, see Fig. 1b for model details) to 40% (highly mineralized collagen fibril). As shown in Fig. 2a, the mineral is deposited predominantly in the gap region along the fibril axis, with sparse deposition in the overlap region. Figure 2b shows a detailed analysis of the distribution of mineral crystals for 20 and 40% mineral densities. We perform a direct comparison with experimental results that reported the distribution of mineral density as a function of the fibril axis26, and plot the mineral distribution of 40% case normalized to its maximum value along with experimental results along the fibril axis (Fig. 2c). In good agreement with a rich set of experimental data1234562627 in the mineralized microfibril models with varying densities (5, 10, 20, 40%, and so on), the mineral deposition occurs primarily in the gap region. This observation is also consistent with the experimental finding26 that the mineral nucleation point is close to the carboxy terminus (in the first section of the gap region, immediately after the gap/overlap transition). As discussed by Nudelman et al.26, the main reason is the high concentration of positive charge, which attract negatively charged peptides, in turn responsible for onset of mineralization. Our in silico mineralization process is not driven by electrostatic interactions (see Methods), but rather relies on a geometric argument, driven by the void space available within the virgin (non-mineralized) collagen fibril. This may suggest that in the in vivo mineralization process there could be concurrent mechanisms that lead to the nucleation in the C-terminal region. Along with the concentration of positive net charges, the C terminus region also features larger void spaces in the collagen packing, which makes this region the preferred site for mineral nucleation. We find that the 40% mineral-density case shows more mineral deposition in the gap region compared with experimental data26. This could be due to the fact that in the experimental work, the mineral density profile is measured for collagen that is mineralized for 24 h. Our model shows that the size of the mineral platelets in the gap region is ~15 × 3 × 1.6 nm3 (for the case of 40% mineral density). This is in good agreement with experimental work that has shown that the size of mineral platelets is 15–55 × 5–25 × 2–3 nm3 (262728). We note that the models are based on a periodic unit cell of a collagen microfibril with approximate dimensions of 67.8 × 4 × 2.7 nm (see further discussion regarding structural aspects below). It is possible that the mineral phase can span several unit cells and, hence, forms larger size HAP platelets in vivo, explaining the larger range of values.

Bottom Line:
Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities.We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils.These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

ABSTRACTBone is a natural composite of collagen protein and the mineral hydroxyapatite. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale. Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities. We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils. Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material's deformation response. These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.